The present invention is used particularly advantageously, albeit without any kind of limitation, in ultra narrow band wireless communication systems. “Ultra Narrow band” (UNB) is understood to mean that the instantaneous frequency spectrum of the radio signals transmitted by the terminals has a frequency width of less than one kilohertz.
Such UNB wireless communication systems are particularly suitable for applications of M2M (Machine-to-Machine) type or of “Internet of Things” (IoT) type.
In such a UNB wireless communication system, data exchanges are essentially unidirectional, in this case on an uplink between terminals and an access network of said system.
The terminals transmit uplink messages that are collected by base stations of the access network, without having to previously associate themselves with one or more base stations of the access network. In other words, the uplink messages transmitted by a terminal are not intended for one specific base station of the access network, and the terminal transmits its uplink messages supposing that they will be able to be received by at least one base station. Such arrangements are advantageous in that the terminal does not need to make regular measurements, which are intensive particularly from the point of view of power consumption, in order to determine the most appropriate base station to receive its uplink messages. The complexity is on the access network, which has to be capable of receiving uplink messages that can be transmitted at arbitrary instants and on arbitrary center frequencies. Each base station of the access network receives uplink messages from the various terminals that are within its range.
Such a mode of operation, in which the data exchanges are essentially unidirectional, is entirely satisfactory for many applications, such as, by way of example, remote reading of gas, water and electricity meters, remote surveillance of buildings or houses, etc.
In some applications, however, it may be advantageous to be able also to exchange data in the other direction, namely on a downlink from the access network to the terminals, for example in order to reconfigure a terminal and/or control an actuator connected to said terminal. However, it is desirable to provide such a capability while limiting the impact on collection of uplink messages.
In particular, in such UNB wireless communication systems, bit rates are low by construction, generally between a few tens of bits per second and a few kilobits per second. Therefore, even if the number of bits to be transmitted in a downlink message is small, the duration of said downlink message may be non-negligible, in the order of one second.
Therefore, it is necessary to limit the number of bits in the downlink messages in order to reduce the duration thereof, and therefore to reduce the duration of occupation of the frequency band used to exchange data between the terminals and the access network.
Such limitation of the number of bits in the downlink messages is also desired on account of the fact that, in order to reduce the cost of deploying the access network, the use of half-duplex base stations is envisaged, that is to say base stations that can receive uplink messages and transmit downlink messages, but not simultaneously. By limiting the duration of the downlink messages, the time during which the base stations cannot receive uplink messages would also be reduced.
Within the context of LTE (Long Term Evolution) wireless communication systems, patent applications US 2013/077583 A1 and WO 2014/075239 A1 disclose the practice of scrambling the data to be transmitted using an identifier of the addressee terminal. The identifier of the addressee terminal is therefore not explicitly transmitted, allowing the number of bits transmitted to be reduced.
However, patent applications US 2013/077583 A1 and WO 2014/075239 A1 rely on the mechanisms of the LTE access network for managing the identifiers of the terminals, which are difficult to implement in an access network in which the aim is to reduce complexity.